1. Field of the Invention
This invention relates to a process of forming a patterned polyimide film by chemical conversion of a polyimide precursor into polyimide. The invention also relates to the manufacture of articles containing such a film, and to the articles themselves.
2. Description of the Prior Art
Polyimide thin layers are useful in many electronic devices and other miniaturized devices, especially those made by thin layer technology such as semiconductor devices, semiconductor memories and circuit boards. Polyimide has the advantage of high thermal stability and low dielectric constant. The known methods for forming a patterned polyimide film are punching, etching and photolithography using photosensitive polyimide.
An example of selective etching of a polyimide film using a mask is disclosed in U.S. Pat. No. 4,436,583. This is a wet etching process. Etching methods can be classified into wet methods and dry methods. In the wet method, the molecular chains of polyimide are dissociated and removed using an alkaline etching solution such as an aqueous solution of NaOH, KOH or hydrazine. The disadvantages of wet etching are that the etching rate varies greatly with different kinds of polyimide, and that there is a risk of delamination of the mask because a long etching time is required when film thickness is large. There is also the disadvantage that the etching liquid contaminates the device being manufactured, and particularly that hydrazine, which is used in the process of U.S. Pat. No. 4,436,583, is highly poisonous to the human body and raises a problem of environmental pollution.
In dry etching, the etching is effected using plasma or ions of oxgyen or a mixture of fluorocarbon and oxygen. Dry etching of polyimide takes a longer time than wet etching and also has the problem that it may damage the device, e.g. a semiconductor element, being formed.
A method involving punching of the polyimide film can be used when forming a flexible printed circuit board or a carrier for a semiconductor device, but has the disadvantages of inferior dimensional accuracy and that a pattern cannot be formed while the polyimide film is located on an electronic device.
Polyimide films for punching or etching have been made by heating a film of polyimide precursor material, to cause formation of the imide rings. Photolithography using photosensitive polyimide also uses heating to form the polyimide ring structure, but forms a pattern of the precursor before the heating stage. A layer of photosensitive polyimide precursor in the form of a polyamic acid having photosensitive functional groups is formed on the substrate and is exposed through a mask. Since the polyimide precursor is photosensitive, a photoresist mask cannot be used, and instead a layer of glass having a metal layer defining the pattern is used as the mask. The radiation used causes cross-linking of photosensitive functional groups in the polyamic acid. Solvent then removes the masked areas which have not been cross-linked, and the remaining pattern is heated to form polyimide.
A problem with this photolithography process is that when the film is thick there is insufficient transmission of light to expose the full thickness of the layer, and even when a thin film is used there is difficulty in forming a fine pattern.
The use of heat to form the polyimide layer is also a disadvantage, if it might cause damage to the product being made.